MIDDLE CLOUDS IN RELATION TO THE JETSTREAM

Jets indicate as much individuality with respect to associated weather as do fronts. Because of the individuality of jetstreams, and also because of the individuality of each situation with respect to humidity distribution and lower level circulation patterns, statistically stated relationships become somewhat vague and are of little value in forecasting.

SHORT-RANGE EXTRAPOLATION

LEARNING OBJECTIVES: Compare short-range extrapolation techniques for the movement of frontal systems and associated weather.

The purpose of this section is to outline several methods that are particularly suited to preparing forecasts for periods of 6 hours or less. The techniques presented are based on extrapolation. Extrapolation is the estimating of the future value of some variable based on past values. Extrapolation is one of the most powerful short-range forecasting tools available to the forecaster.

Nephanalysis may be defined as any form of analysis of the field of cloud cover and/or type. Cloud observations received in synoptic codes permit only a highly generalized description of the actual structure of the cloud systems.

Few forecasters make full use of the cloud reports plotted on their surface charts, and often, the first consideration in nephanalysis is to survey what cloud information is transmitted, and to make sure that everything pertinent is plotted. For very short-range forecast, the charts at 6-, 12-, and 24-hour intervals are apt to be insufficient for use of the extrapolation techniques explained in this chapter. Either nephanalysis or surface charts should then be plotted at the intermediate times from 3-hourly synoptic reports or even from hourly sequences. An integrated system of forecasting ceiling, visibility, cloud cover, and precipitation should be considered simultaneously, as these elements are physically dependent upon the same synoptic processes. With present-day satellite capabilities, it is rare that a nephanalysis would be manually performed. Instead, the surface analysis and satellite imagery will be used together.

For short-range forecasting, the question is often not whether there will be any precipitation, but when will it begin or end.

This problem is well suited to extrapolation methods. For short-range forecasting, the use of hourly nephanalyses often serve to "pickup" new precipitation areas forming upstream in sufficient time to alert a downstream area. Also, the thickening and lowering of middle cloud decks generally indicate where an outbreak of precipitation may soon occur.

Forecasting the Movement of Precipitation Areas by Isochrones

The areas of continuous, intermittent, and showery precipitation can be outlined on a large-scale 3-hourly or hourly synoptic chart in a manner similar to the customary shading of precipitation areas on ordinary synoptic surface weather maps. Different types of lines, shading, or symbols can distinguish the various types of precipitation. Isochrones of several hourly past positions of the lines of particular interest can then be added to the chart, and extrapolations for several hours made from them if reasonably regular past motions are in evidence. A separate isochrone chart (or acetate overlay) may be easier to use. Lines for the beginning of continuous precipitation are illustrated in figure 4-12. The isochrones for showery or intermittent precipitation usually give more uncertain and irregular patterns, which result in less satisfactory forecasts. When large-scale section surface weather maps are regularly drawn, it maybe sufficient and more convenient to make all precipitation area analyses and isochrones on these maps.

Forecasting the Movement of Precipitation by Using a Distance versus Time (x-t) Diagram

The idea of plotting observations taken at different times on a diagram that has horizontal or vertical distance in the atmosphere as one coordinate and time as the other has been used in various forms by forecasters for years. The time cross section that was discussed in the AG2 TRAMAN, volume 1, unit 9, lesson 2. is a special case of this aid, where successive information at only one station is plotted.

LOWERING OF CEILING IN CONTINUOUS RAIN AREAS

One of the many obstacles the forecaster faces in preparing forecasts is the problem of determining "when" ceilings heights will lower in areas expecting rain. In the following paragraphs, we will discuss this dilemma.

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